Wet wipes are sheets of fabric stored in a solution prior to use and normally used to wipe the skin. The most common types of wet wipes are baby wipes, typically used to clean the seat area during a diaper change, and adult wipes, used to clean hands, face and bottom. Wet wipes are often made from bonded nonwoven fabrics that have sufficient tensile strength that they will not fall apart during manufacturing or in use, yet have desirable softness characteristics for use on skin in tender areas. Such nonwoven fabrics are commonly manufactured by meltspun processes, such as meltblown and spunbond processes, known to those skilled in the art, because meltspun fabrics can be produced that have the requisite tensile strength and softness.
Bonding of nonwoven materials generally builds strength and integrity in nonwoven fabrics. Many conventional bonding systems are used to make nonwoven fabrics, such as, but not limited to, thermal bonding, resin bonding (aqueous or melt), hydroentanglement, and mechanical bonding. These broad classifications can be subdivided into overall treatment or zone treatment such as dots, lines or small areas of patterns. Further, the degree of bonding can be controlled. A high degree of bonding by higher percentage add on or higher energy input usually builds higher strengths and vice versa. However, bonding normally negates the ability for post-use disposal by disintegration and dispersion during toilet flushing.
Many of the items or products into which bonded meltspun materials are incorporated are generally regarded as being limited use disposable products. By this it is meant that the product or products are used only a limited number of times and in some cases only once before being discarded. With increasing concerns over solid waste disposal, there is now an increasing need for materials that are, for example, either recyclable or disposable through other mechanisms besides incorporation into landfills. One possible alternative means of disposal for many products, especially in the area of personal care absorbent products and wipers, is by flushing them into sewage disposal systems. As will be discussed in greater detail below, flushable means that the material must not only be able to pass through a commode without clogging it, but the material must also be able to pass through the sewer laterals between a house (or other structure housing the commode) the main sewer system without getting caught in the piping, and to disperse into small pieces that will not create a nuisance to the consumer or in the sewer transport and treatment process.
In recent years, more sophisticated approaches have been devised to impart dispersability. Chemical binders that are either melt processable or aqueous and emulsion processable have been developed. The material can have high strength in their original storage environment, but quickly lose strength by debonding or dispersing when placed in a different chemical (e.g., pH or ion concentration) environment, such as by flushing down a commode with fresh water. It would be desirable to have a bonding system that would produce a fabric having desirable strength characteristics, yet be able to disperse or degrade after use into small pieces. As machines for producing such bonded nonwoven fabrics are usually designed to work with one bonding system, hybrid bonding systems are generally unknown in the industry.
U.S. Pat. Nos. 4,309,469 and 4,419,403, both issued to Varona describe a dispersible binder of several parts. Reissue Patent no. 31,825 describes a two-stage heating process (preheat by infrared) to calendar bond a nonwoven consisting of thermoplastic fibers. Although offering some flexibility, this is still a single thermal bonding system. U.S. Pat. No. 4,207,367 issued to Baker, describes a nonwoven which is densified at individual areas by cold embossing. The chemical binders are sprayed on and the binders preferentially migrate to the densified areas by capillary action. The non-densified areas have higher loft and remain highly absorbent. However, it is not a hybrid bonding system because the densification step is not strictly a bonding process. U.S. Pat. No. 4,749,423, issued to Vaalburg et al., describes a two stage thermal bonding system. In the first stage, up to 7% of polyethylene fibers in a web is fused to provided temporary strength to support transfer to the next stage. In the second stage the primary fibers are thermally bonded to give the web its overall integrity. This process in two distinct stages does not make the web have built in areas of strength and weakness. It is not suitable as a dispersible material.
Several patents describe hybrid bonding systems, but are for sanitary napkin covers. For example, see U.S. Pat. No. 3,654,924, to Duchane, U.S. Pat. No. 3,616,797, issued to Champagne et al., and U.S. Pat. No. 3,913,574, issued to Srinvasan et al. The important difference is that these products are designed to be stored dry and to have very limited wet strength for a short duration during use. In a wet wipe there remains a need for prolonged wet strength in a storage solution.
Fibrous nonwoven materials and fibrous nonwoven composite materials are widely used as products or as components of products because they can be manufactured inexpensively and can be made to have specific characteristics. One approach has been to mix thermoplastic polymer fibers with one or more types of fibrous material and/or particulates. The mixtures are collected in the form of fibrous nonwoven web composites which may be further bonded or treated to provide coherent nonwoven composites that take advantage of at least some of the properties of each component. For example, U.S. Pat. No. 4,100,324 issued Jul. 11, 1978, to Anderson et al. discloses a nonwoven fabric which is generally a uniform admixture of wood pulp and meltblown thermoplastic polymer fibers. U.S. Pat. No. 3,971,373 issued Jul. 7, 1976, to Braun discloses a nonwoven material which contains meltblown thermoplastic polymer fibers and discrete solid particles. According to this patent, the particles are uniformly dispersed and intermixed with the meltblown fibers in the nonwoven material. U.S. Pat. No. 4,429,001 issued Jan. 31, 1984, to Kolpin et al. discloses an absorbent sheet material which is a combination of meltblown thermoplastic polymer fibers and solid superabsorbent particles. The superabsorbent particles are disclosed as being uniformly dispersed and physically held within a web of the meltblown thermoplastic polymer fibers. European Patent Number 0080382 to Minto et al. published Jun. 1, 1983, and European Patent Number 0156160 to Minto et al. published Oct. 25, 1985, also disclose combinations of particles such as superabsorbents and meltblown thermoplastic polymer fibers. U.S. Pat. No. 5,350,624 to Georger et al. issued Sep. 27, 1994, discloses an abrasion-resistant fibrous nonwoven structure composed of a matrix of meltblown fibers having a first exterior surface, a second exterior surface and an interior portion with at least one other fibrous material integrated into the meltblown fiber matrix. The concentration of meltblown fibers adjacent to each exterior surface of the nonwoven structure is at least about 60 percent by weight and the concentration of meltblown fibers in the interior portion is less than about 40 percent by weight. Many of the aforementioned admixtures are referred to as "coform" materials because they are formed by combining two or more materials in the forming step into a single structure. Coform materials can also be produced by a spunbond process, such as is disclosed in U.S. Pat. No. 4,902,559 to Eschwey et al. issued Feb. 20, 1990.
Currently, one common method of meltblown formation of coform nonwoven material involves injecting an amount of cellulose fibers or blends of cellulose fibers and staple fibers into a molten stream of meltblown fibers. Coform material injected into the fiber stream becomes entrapped or stuck to the molten fibers, which are subsequently cooled or set. In a further step the fabric can be bonded by thermally or ultrasonically melting the meltblown fibers to cross-bond the fibers together, imparting desired tensile strength. Such bonding treatment also reduces softness because it reduces freedom of movement between the meltblown fibers in the web structure. Thus, the imparting of strength has, heretofore resulted in a diminution of softness (absent additional steps of softening, which affect material properties and add to production costs). Moreover, because the meltblown fibers are preferentially used in water dispersible fabrics because of the low denier fiber produced, fiber strength is compromised. It would be desirable to produce a fabric having desirable strength and softness characteristics, yet be water dispersible.
Coform engineered composites can be used in a wide variety of applications including absorbent media for aqueous and organic fluids, filtration media for wet and dry applications, insulating materials, protective cushioning materials, containment and delivery systems and wiping media for both wet and dry applications. Many of the foregoing applications can be met, to varying degrees, through the use of more simplified structures such as absorbent structures wherein only wood pulp fibers are used. This has commonly been the case with, for example, the absorbent cores of personal care absorbent products such as diapers. Wood pulp fibers when formed by themselves tend to yield nonwoven web structures which have very little mechanical integrity and a high degree of collapse when wetted. The advent of coform structures which incorporated thermoplastic meltblown fibers, even in small quantities, greatly enhanced the properties of such structures including both wet and dry tensile strength. The same enhancements were also seen with the advent of coform wiping sheets.
The very reason why many coform materials provide increased benefits over conventional materials, i.e., the meltblown thermoplastic fiber matrix, is the same reason why such materials are more difficult to recycle or flush. Many wood pulp fiber-based products can be recycled by hydrating and repulping the reclaimed wood pulp fibers. However, in coform structures the thermoplastic meltblown fibers do not readily break-up. The meltblown fibers are hard to separate from the wood pulp fibers, and they remain substantially continuous thereby giving rise to the possibility of clogging or otherwise damaging recycling equipment such as repulpers. From the standpoint of flushability, the current belief is that to be flushable, a product must be made from very small and/or very weak fibers so that the material will readily break-up into smaller pieces when placed in quantities of water such as are found in toilets and, again due to the nature of the fibers, when flushed will not be entrained or trapped within the piping of conventional private and public sewage disposal systems. Many of these systems, especially sewer laterals, may have many protrusions within the pipes such as tree roots which will snag any type of material which is still relatively intact. Such would be the case with conventional non-water-dispersible meltblown thermoplastic fibers in coform materials. As a result, for at least the foregoing reasons, there is a need for a coform material which has the potential for being more user friendly with respect to recycling processes and disposal through alternative means to landfills such as, for example, flushing. Accordingly, it is an object of the present invention to provide such a material.